The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This disclosure relates to compositions and methods for treating subterranean formations, in particular, compositions and methods for cementing subterranean wells.
During the construction of subterranean wells, it is common, during and after drilling, to place a tubular body in the wellbore. The tubular body may comprise drillpipe, casing, liner, coiled tubing or combinations thereof. The purpose of the tubular body is to act as a conduit through which desirable fluids from the well may travel and be collected. The tubular body is normally secured in the well by a cement sheath. The cement sheath provides mechanical support and hydraulic isolation between the zones or layers that the well penetrates. The latter function is important because it prevents hydraulic communication between zones that may result in contamination. For example, the cement sheath blocks fluids from oil or gas zones from entering the water table and polluting drinking water. In addition, to optimize a well's production efficiency, it may be desirable to isolate, for example, a gas-producing zone from an oil-producing zone. The cement sheath achieves hydraulic isolation because of its low permeability. In addition, intimate bonding between the cement sheath and both the tubular body and borehole is necessary to prevent leaks.
Portland cement is employed to cement the vast majority of subterranean wells. Achieving optimal cement-slurry placement and set-cement properties usually requires the incorporation of one or more additives that modify the chemical and/or physical behavior of the slurry. A plethora of additives exist that fall into several categories including (but not limited to) accelerators, retarders, dispersants, fluid-loss additives, extenders, pozzolans, weighting agents, swellable materials, gas-generating materials, and antifoam agents. An extensive discussion concerning additives for well cements may be found in the following publication—Nelson E B, Michaux M and Drochon B: “Cement Additives and Mechanisms of Action,” in Nelson E B and Guillot D. (eds.): Well Cementing (2nd Edition), Schlumberger, Houston (2006) 49-91.
Designing cement slurries for high-pressure, high-temperature (HPHT) wells is particularly challenging. Generally speaking, HPHT wells begin when the bottomhole temperature exceeds about 150° C. (300° F.) and the bottomhole pressure exceeds about 69 MPa (10,000 psi). A complex array of additives—including retarders, dispersants, fluid-loss additives and silica stabilizers—is usually required to obtain a slurry that operators can successfully place in the well, and a set-cement that will provide casing support and zonal isolation throughout the life of the well.
Weighing agents are also frequently employed in cement slurries for HPHT wells. High-density slurries are required to exert sufficient hydrostatic pressure in the wellbore to maintain well control. One method for increasing the cement-slurry density is to reduce the amount of mix water. To maintain pumpability, the addition of a dispersant is required. The principal disadvantage of such reduced-water slurries is the difficulty of simultaneously achieving adequate fluid-loss control, acceptable rheological properties and slurry stability (i.e., no solids settling). Generally, the maximum slurry density attainable by reducing the mix-water concentration is about 2160 kg/m3 (18.0 lbm/gal).
Many HPHT wells require higher slurry densities. Under these circumstances, materials with a high specific gravity (known as weighting agents) are added. Such materials must meet several criteria to be acceptable as weighting agents. The particle-size distribution of the material must be compatible with the cement. Large particles tend to settle out of the slurry, while small particles tend to increase slurry viscosity. The mix-water requirement must be low (i.e., very little water should be necessary to wet the weighting-agent particles). The material must be inert with respect to the cement, and must be compatible with other additives in the cement slurry.
The most common weighting agents for Portland-cement slurries are hematite (Fe2O3), ilmenite (FeTiO3), hausmannite (Mn3O4) and barite (BaSO4). Their physical properties are given in Table 1. The specific gravities may vary from batch to batch owing to impurities that may be present.
TABLE 1Physical Properties of Weighting Agents for Cement Slurries.AdditionalWaterWeightingSpecificAbsoluteRequirementAgentGravityVolume (L/kg)Color(L/kg)Hematite4.450.201Black0.019Ilmenite4.950.225Red0.000Hausmannite4.840.209Reddish brown0.009Barite4.330.234White0.201
Large particles with a high specific gravity have a strong tendency to settle. As shown by Stoke's law (Eq. 1), the settling velocity of a particle is more dependent on its size than on its specific gravity.
                    v        =                              g            ×                          (                              ρ                -                                  ρ                  L                                            )                        ×                          d              2                                            18            ⁢                          μ              L                                                          (                  Eq          .                                          ⁢          1                )            
where:                ν=settling velocity        g=acceleration of gravity        ρ=particle specific gravity        ρL=liquid specific gravity        d=particle diameter        μL=liquid-medium viscosity.        
For example, the specific gravities of hematite and silica sand are 4.95 and 2.65, respectively. According to Stoke's law, for a given particle size, the hematite particle would settle about twice as fast as the silica particle. However, for a given particle density, if the particle size is increased to 500 μm from 1 μM, the settling rate increases by a factor of 250,000.
Stoke's law clearly shows that the size of the solid materials added to a cement slurry should preferably be low in order to minimize settling problems. Thus, the use of weighting agents with very fine particle-size distributions would generally enhance the stability of cement slurries.
Barite is commercially available in several particle-size distributions, but it is not considered to be an efficient weighting agent compared to hematite, ilmenite or manganese tetraoxide. It has a lower specific gravity, and requires a significant amount of additional water to wet its particles—further diminishing its effectiveness as a weighting agent. Therefore, although it is commonly used in drilling fluids and spacer fluids, barite is seldom used in cement slurries.
With a specific gravity of 4.95, hematite is an efficient weighting agent and is routinely used in the industry. It is usually supplied with a fine particle size distribution, with a median particle size of about 30 μm. To the inventors' knowledge, the only commercially available weighting agent with a finer particle size is Micromax™, manufactured by Elkem AS, Oslo, Norway. It is composed of hausmannite with a median particle size of about 2 μm.
Until recently, it has been assumed that barite, ilmenite, hematite and hausmannite are inert with respect to Portland cement hydration and the set cement. However, the inventors recently discovered that, at high temperatures, hematite and hausmannite are not inert. At this temperature, the calcium-silicate-hydrate mineral xonotlite (6CaO.6SiO2.H2O) is usually the principal binding phase in set Portland cement that has been stabilized with silica. Hematite and hausmannite react with xonotlite to form other minerals, including andradite (Ca2Fe2Si3O12) and calcium manganese silicates such as johannsenite (CaMnSi2O6). Formation of these minerals is accompanied by a reduction of the cement compressive strength and an increase of cement permeability. Such an effect is potentially detrimental to the set cement's ability to provide zonal isolation.
Therefore, in the context of HPHT wells, it would be desirable to have weighting agents that are inert with respect to calcium-silicate-hydrate minerals in set Portland cement, and do not have a deleterious effect on the physical properties of set Portland cement.